A foundry process widely used for making cores and molds entails the sulfur dioxide (SO2) cured epoxy-acrylate binder system. In this process, a mixture of a hydroperoxide (usually cumene hydroperoxide), an epoxy resin, a multifunctional acrylate, a silane coupling agent, and optional diluents, are mixed with an aggregate (typically sand) and compacted into a pattern to give it a specific shape. The confined mixture is contacted with SO2 vapor, optionally diluted with nitrogen, by blowing the SO2 into the pattern where the shape is contained. There, the SO2 reacts with the hydroperoxide to form an acid and free radicals. The generated acid cures the epoxy resin and the generated free radicals cure the multifunctional acrylate. The mixture is instantaneously hardened to result in the desired shape and can be used immediately in a foundry core and/or mold assembly.
The epoxy-acrylate binders used in this process are currently sold by Ashland Specialty Chemical under the trade name of ISOSET® and ISOSET THERMOSHIELD™ binders. Though the process has been used successful in many foundries, one of the major weaknesses of the epoxy-acrylate binder system has been the lack of adequate erosion resistance. Erosion occurs when molten metal contacts the mold or core surfaces during the pouring process and sand is dislodged at the point of contact. This occurs because the binder does not have sufficient heat resilience to maintain surface integrity until the pouring process is complete. The result is that loose sand is carried into the mold cavity by the liquid metal, creating sand inclusions and weak areas in the casting. A dimensional defect is also created on the surface of the casting.
To correct this problem, foundries have historically resorted to the use of refractory coatings. Core and mold assemblies or parts thereof are dipped into, flowed or sprayed with a slurry consisting of a high melting refractory oxide, a carrier such as water or alcohol, and thixotropic additives. When dried on a mold or core surface, the coating very effectively prevents erosion, in most cases. The problem with this approach is that the coating operation is messy, adds complexity to the sand casting process, and requires expensive gas fired, microwave, or radiant energy ovens to dry the wash onto the core surface. When the core and/or molds are heated during the drying process, the strength of the organic binder-to-aggregate bond weakens significantly. This results in problems handling the hot cores and reduction in productivity due to distortion or cracking of the core or mold.
All citations referred to under this description of the “Background” and in the “Detailed Description” of the invention are expressly incorporated by reference.